This invention lies in the field of seismic prospecting. More particularly it is concerned with seismic operations in which a long time duration vibrator input signal is imposed on the earth. Still more particularly it is concerned with vibrator operations responsive to a sweep signal of selected frequency content and time duration, in which the vibrator is driven directly from the sweep signal without the use of any auxiliary control to maintain a selected phase relation between the output of the vibrator and the sweep signal.
In the prior art, the geophysical industry has used long time duration vibrator signals to generate seismic waves in the earth. This has generally been done with multiple vibrators, usually from three or four, to as many as ten or twelve, per crew. These vibrators are placed in close proximity to each other and are driven by the same reference or sweep signal, in time and phase synchronism with each other. In order to provide this synchornism between each of the multiple vibrators, it is necessary to maintain the output of each vibrator in time and phase synchronism with its own driving sweep signal.
There is a large amount of prior art devoted to the operation of driving a vibrator from an electrical sweep signal, in conjunction with special control signals, so that the output of the vibrator will always be inphase with the electrical sweep signal. There are a number of patents issued covering apparatus designed for this purpose. Among such U.S. Nos. is 3,208,545, issued Sept. 28, 1965 in the name of W. E. N. Doty et al, entitled "Apparatus for Controlling a Seismic Vibrator," and 3,698,508, issued Oct. 17, 1972, in the name of R. W. Landrum, entitled "Phase Control of Servo Hydraulic Vibrators," and others.
In the prior art, sweep signals have been provided at the vibrator location by several means:
1. by analog radio transmission to the vibrator location from the recording location in the form of an analog sweep signal,
2. by magnetic playback of an analog signal,
3. by generating at the vibrator location a digital sweep signal and converting this digital signal to an analog signal to drive the vibrator.
The control system that serves to maintain the phase synchronism has almost exclusively been analog instrumentation, similar to that shown in the Doty patent. This generally comprises analog processing means including integrators, amplifiers, multipliers, etc. to take the output of a sensor on the vibrator base plate, and provide a signal (called for convenience, "transmitted" signal) corresponding to the seismic wave generated in the earth. It also comprises comparison means to determine an error signal between the sweep signal and the transmitted signal. And finally there is a phase shift means responsive to the error signal to convert the analog sweep signal into a phase shifted version of the sweep signal, called the drive signal, which is used to drive the vibrator.
All of this analog equipment is required at each of the vibrators solely for the purpose of maintaining a selected phase between the output of the vibrator and the sweep signal. It is costly equipment, and being analog equipment is not the optimum form of instrumentation for accomplishing the desired purpose.